The most important problem the automobile industry is faced with is the reduction of fuel consumption rate. This increasingly requires the reduction of the weight of tires.
Currently, the inside of tires has an inner liner or an air permeation preventing layer that is made of a halogenated butyl rubber or another rubber having low air permeation.
However, the halogenated butyl rubber employed as an inner liner or an air permeation preventing layer has a high hysteresis loss, causing a ripple on the inner rubber of a carcass layer and the air permeation preventing layer after a vulcanization of the tire and thereby deforming both the carcass layer and the air permeation preventing layer. This leads to an increased rolling resistance.
As a solution of this problem, a rubber sheet called “tie rubber” having a low hysteresis loss is inserted between the air permeation preventing layer (halogenated butyl rubber) and the carcass layer. The insertion of the rubber sheet increases the total thickness of the tire layer above 1 mm (1,000 μm) in addition to the thickness of the air permeation preventing layer made of a halogenated butyl rubber. This results in the increased weight of the complete tire.
In an attempt to solve this problem, there have been suggested techniques of employing different materials for the air permeation preventing layer of pneumatic tires instead of such a conventional rubber material as halogenated butyl rubber having low air permeation.
For example, Japanese Patent Laid Open No. 6-40207 proposes a technique of providing an air permeation preventing layer in the inside of the tire by laminating a low air permeation layer including a polyvinylidene chloride film or an ethylene-vinyl alcohol copolymer film and an adhesive layer including a polyolefin film, an aliphatic polyamide film, or a polyurethane film to form a thin film, affixing the thin film to the inner side of the green tire consisting of non-vulcanized rubber to make the adhesive layer in contact with the carcass layer, and then vulcanizing and shaping the green tire.
The use of a thin air permeation preventing layer makes it possible to reduce the weight of the tire without deteriorating the maintenance of pneumatics.
However, the thermoplastic multi-layer film, if used for an inner liner or another air permeation preventing layer, has a low elongation with respect to repeated deformations while in use, causing a lot of cracks on it and hence a deterioration of air tightness.
In the conventional tire manufacturing method that requires a step of shaping an inner liner, the thermoplastic film commercially available encounters oriented crystallization caused by drawing and heat crystallization by thermosetting after drawing and annealing processes, providing poor elongation against deformation in the shaping process with a consequence of fracture. In conclusion, the conventional manufacturing methods make it impossible to realize the fabrication of tires from thermoplastic films commercially available.